1. Field of the Invention
The present invention relates to graphite materials which are well suited to negative electrode materials for use in a lithium secondary battery, and which are capable of enhancing the performance of the battery. More particularly, the present invention pertains to graphite materials for a negative electrode used in a lithium secondary battery which materials are capable of enhancing the performance of the battery by a technique in which milled graphite fibers and specific graphite materials (natural graphite, artificial graphite or a mixture thereof) are mixed with each other at a an appropriate proportion so as to regulate the specific surface area of the mixture within a specific range.
2. Description of the Related Art
There has been widely investigated and put into practice the use of mesophase pitch-based milled graphite fibers as a negative electrode for a lithium secondary battery, as disclosed for example, in Japanese Patent Application Laid-Open Nos. 325967/1993 (Heisei-5), 36802/1994 (Heisei-6), and 90725/1995 (Heisei-7).
The mesophase pitch-based milled graphite fibers are excellent in that the fibers have high efficiency of initial charge and discharge, good load characteristics and further favorable adhesivity to electricity collectors. However, when compared with natural graphite and artificial graphite which have a high degree of graphitization, the aforesaid fibers are somewhat inferior with respect to initial capacity and low rate capability presumably because of their lower degree of graphitization. Accordingly, it is considered to be some room for their further improvement.
On the one hand, there has been investigated and put into practice in part, the use of natural graphite and artificial graphite which have a high degree of graphitization as negative electrode materials for a lithium secondary battery. Nevertheless, there is recognized the tendency that these graphite has low efficiency of initial charge and discharge, poor load characteristics, further unfavorable adhesivity to electricity collectors and the like as compared with the mesophase pitch-based milled graphite fibers.
In addition, for the purpose of complementing the items to be improved in a single kind of material, researches have been made on the mixed use of various materials.
In regard to the mixing system, the scope of researches covers wide fields including a variety of combinations of the materials to be mixed, such as graphite base/graphite base, carbon base/carbon base, graphite base/carbon base, graphite base/carbon base that are incorporated with an electroconductive material and a mixed system of at least three kinds of materials.
With respect to the mixing of graphite base/graphite base, for example, there is disclosed that improvements are recognized in the capacity and cycle characteristics on the discharge per hour by the mixing of spheroidal graphitized carbon particles and 5 to 30% by weight of graphitized carbon short fiber in Japanese Patent Application Laid-Open No.111818/1994 (Hesei-6).
There are also disclosed for the same purpose, the mixing of graphite materials comprising spheroidal particles (MCMB) and not more than 20% by weight of graphite fine particles (natural graphite, artificial graphite or the like) having a particle diameter smaller than that of MCMB in Japanese Patent Application Laid-Open No. 37618/1995 (Heisei-7); the mixing of graphite materials comprising spheroidal particles (MCMB) or the like and 5 to 20% by weight of fibrous graphite in Japanese Patent Application Laid-Open No. 287952/1996 (Heisei-8).
There are also disclosed for the same purpose, the mixing of mesophase pitch-based milled carbon fibers and 5 to 20% by weight of natural or artificial graphite in Japanese Patent Application Laid-Open No. 83608/1996 (Heisei-8); and the mixing of graphite material and graphite material different in bulk density from each other in Japanese Patent Application Laid-Open No. 180873/1996 (Heisei-8).